A popular dictionary defines “ventilation” as the admission of “fresh air . . . to replace stale or noxious air”. In the context of a building, this means introducing outdoor air and circulating it throughout the building to dilute contaminants and odors. The amount of air needed for proper ventilation largely depends on the population of building. In most commercial applications, the number of people in the building seldom equals the design occupancy. As a result, at least some spaces in the building are over ventilated leading to correspondingly higher than necessary energy costs.
Static rate ventilation is a control strategy where a particular ventilation rate is set and the particular rate does not vary to meet changing conditions.
Various demand controlled ventilation strategies attempt to control ventilation based upon actual occupancy and reduce operating costs by optimizing or at least controlling the rate of outdoor air intake to something less than maximum capacity.
Demand controlled ventilation describes control strategies that respond to the actual demand (need) for ventilation by regulating the rate at which the HVAC system brings outdoor air into the building. There are several such strategies: occupancy sensors detecting the presence or number of people in each monitored zone; occupancy schedules which allow a building automation system to schedule the current population based on the time of day; and carbon dioxide sensors monitoring the concentration of carbon dioxide that the occupants continuously produce.
The practice of using carbon dioxide concentrations as an indicator of population or ventilation rate is often called CO2 Based Demand Controlled Ventilation. A general description of such a strategy is described in “Using CO2 for Demand Controlled Ventilation”, Trane Engineering Newsletter, Volume 31, Number 3, published in September of 2002.
Previous systems select one of the aforementioned strategies and control ventilation for a building or space based upon the selected strategy. Problems in these systems occur when CO2 or occupancy sensors fail or provide invalid signals or when occupancy schedules are not updated or entered correctly.